1. Field of the Invention
The present invention relates to a radiation imaging system for taking a continuous radiographic image, a method for taking the continuous radiographic image, and a radiation image detecting device.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is widely known. The X-ray imaging system is constituted of an X-ray generation apparatus for generating the X-rays, and an X-ray image capturing apparatus for taking an X-ray image of a patient's body. The X-ray generation apparatus includes an X-ray source for emitting the X-rays to the body, a source controller for controlling the operation of the X-ray source, and an exposure switch for issuing an emission start signal of the X-rays. The X-ray image capturing apparatus includes an X-ray image detecting device for detecting the X-ray image based on the X-rays passed through the patient's body, and a console for controlling the operation of the X-ray image detecting device and applying various image processes to the X-ray image.
In recent years, the X-ray image detecting device that uses a flat panel detector (FPD) instead of an X-ray film or an imaging plate (IP) becomes widespread. The FPD has a matrix of pixels each of which accumulates signal charge by an amount corresponding to the amount of the X-rays incident thereon. The FPD accumulates the signal charge on a pixel-by-pixel basis, and converts the accumulated signal charge into a voltage signal by its signal processing circuit. Thereby, the FPD electrically detects the X-ray image, and outputs the X-ray image as digital image data.
An electronic cassette (portable X-ray image detecting device) that has the FPD contained in a flat and thin housing is in practical use. The electronic cassette is mounted not only on a specific imaging support, but also on an existing imaging support shareable between a film cassette and an IP cassette. Furthermore, the electronic cassette can be used while being put on a bed or held by the patient himself/herself, to take a radiograph of a body part that is hard to take with the stationary X-ray image detecting device. The electronic cassette is sometimes brought out from a hospital for use in bedside radiography of a home-care patient or in an outside accident or natural disaster site in an emergency.
There are some X-ray imaging systems having an automatic exposure control (AEC) function. Such systems have a dose detection sensor for detecting a dose of the X-rays passed through the patient's body. When an integration value of the X-ray dose detected by the dose detection sensor reaches a predetermined threshold value, the emission of the X-rays from the X-ray source is stopped.
Japanese Patent Laid-Open Publication No. 2002-000590 discloses an X-ray imaging system that carries out the AEC based on pixel signals from the FPD. This system obtains data that represents a pixel area (irradiation area) of the FPD corresponding to a body portion of interest (body portion to be imaged), that is, the addresses, size, and the like of the pixels corresponding to the body portion. The pixels to be used in the AEC are chosen in accordance with the body portion of interest.
According to Japanese Patent Laid-Open Publication No. 08-033621, in a mammography system, an outline of a breast is recognized in a preliminary exposure, and the size of the breast is calculated. The size of the pixel area (irradiation area) used in the AEC is varied in accordance with the size of the breast.
Besides taking a single exposure of a body part such as chest or abdomen with use of the fixed X-ray source and the fixed X-ray imaging detecting device, continuous radiography is known in which a plurality of exposures are taken while shifting the X-ray source and the X-ray image detecting device to obtain a continuous X-ray image. The continuous radiography is mostly used for observation of bones of the patient, such as skeletal age and bone curvature. In most cases, the continuous radiography is performed on a full spine extending from a clavicle to a pelvis, and on lower limbs extending from the pelvis to toes.
In the continuous radiography, a radiological technician firstly sets up a total image capture field in accordance with a body part to he imaged and the size of the patient i.e. standing height, sitting height, and an inseam. Based on the total image capture field and the size of the X-ray image detecting device (size of an imaging surface of a detection panel), the number of division exposures to be performed and division exposure positions, which are the positions of the X-ray source and X-ray image detecting device set in each division exposure, are calculated. After that, the division exposures are carried out. The division exposure positions are determined such that a plurality of division X-ray images obtained by the division exposures partly overlap one another. After completion of the division exposures, the plurality of division X-ray images are merged into the single continuous X-ray image by overlaying overlap areas of the division X-ray images.
In the continuous radiography, the total image capture field is large in size. Thus, for example, in the case of lower limb radiography, body thickness and the size of a directly-exposed area, in which no object exists, widely vary between waist and leg. Therefore, it is difficult to optimize the X-ray dose in each of the division exposure positions. Accordingly, in Japanese Patent Laid-Open Publication No. 2011-139761, an object is imaged by a digital camera, and an outline of the object is extracted. An AEC irradiation area is set up based on the outline in each division exposure. The pixels of the FPD are used as AEC sensors, and the AEC is performed in each division exposure based on signals from the pixels within the AEC irradiation area.
However, in the mammography system of the Japanese Patent Laid-Open Publication No. 08-033621, the preliminary exposure is carried out with the sole purpose of determining the irradiation area. In the system of the Japanese Patent Laid-Open Publication No. 2011-139761, the outline of the object is extracted from the image of the digital camera. Both the systems bring about increase in size and cost and a complicated and long-time process.
Also, in the Japanese Patent Laid-Open Publication No. 08-033621, the patient has to be exposed to an extra radiation dose in the preliminary exposure. In the Japanese Patent Laid-Open Publication No. 2011-139761, since the irradiation area is determined based on the outline of the object, the irradiation area includes not only an area of an important bone but also an area without the bone. Thus, the AEC may be performed improperly. If the patient puts on clothes or in plaster, the outline of the object becomes unclear, so the irradiation area possibly includes an area of no concern for diagnosis.